The present invention relates to a photonic crystal and a technique of an optical device having, as a component, a photonic crystal in which a waveguide is formed.
When electromagnetic fields are incident on the interface of two media of different refractive indices, generally, a part of the electromagnetic fields is reflected. It is known that the reflected wave can be reduced by an antireflection coating on the interface.
In the case where an electromagnetic field enters a medium 2 having a refractive index n2 from a medium 0 having a refractive index n0 (in the air or vacuum), the interface between the media 0 and 2 is coated with a medium 1 having a refractive index n1 satisfying the relation of n0<n1<n2, and the thickness h1 of the medium 1 is set so as to satisfy the equation of n1h1=(2m+1)λ0/4 (where λ0 denotes a wavelength in vacuum of the incident electromagnetic field and m=0, 1, 2, . . . ). Consequently, a wave reflected by the interface between the media 0 and 1 and a wave reflected by the interface between the media 1 and 2 interfere with each other and, as a result, the reflectance on the incident side becomes R=(n0n2−n12)2/(n0n2+n12)2 (refer to, for example, “Applied Optics II” by Tadao Tsuruta, Baifu-kan, 1990, p116).
The photonic crystal is a medium in which a periodic structure of about a wavelength of an electromagnetic field used is fabricated by a microfabrication technology or the like, and a propagation characteristic of the electromagnetic field depends on the periodic structure. When the electromagnetic field enters the photonic crystal, a part of the electromagnetic field is reflected due to a refractive index difference between the outside and inside of the photonic crystal.
In a manner similar to a normal medium, when the photonic crystal can be antireflection coated, a reflection loss can be reduced. However, there is a case that a normal coating process cannot be performed due to a characteristic structure of the photonic crystal, so that incident light from an optical fiber or the like directly enters a photonic crystal device (refer to, for example, Japanese Patent Application Laid-Open No. 11-218627 and U.S. Pat. No. (USP) 6,075,915, U.S. Pat. No. 6,093,246, U.S. Pat. No. 6,028,693, U.S. Pat. No. 5,907,427, and U.S. Pat. No. 5,751,466).
In a photonic crystal integrated device in which photonic crystal devices are successively disposed (refer to, for example, Japanese Patent Application Laid-Open No. 2000-56146 and U.S. Pat. No. 6,278,105 B1), generally, reflection takes place in a connection part of devices. Since the connection part of integrated devices cannot be antireflection coated in the normal way, it is impossible to prevent the reflection.